[llvm] [RegisterCoalescer] Fix SUBREG_TO_REG handling in the RegisterCoalescer. (PR #96839)

[Stefan Pintilie via llvm-commits]

stefanp-ibm wrote:

> I think we should pre-commit the X86 test case to show where failure occurs.

I would like to do this but the test actually hits an assert so we don't actually generate any code for the X86 test unless I remove the `-verify-coalescing` option which shows the actual failure. 

https://github.com/llvm/llvm-project/pull/96839